The present invention relates in general to a D/A converter and in particular to a D/A converter that can be used well in a digital video camera apparatus or the like.
In general equipment such as a digital video camera apparatus, digital video signal data may be output as an analog video signal. In this case, the digital video camera apparatus may employ a D/A converter for converting digital luminance signal data of the digital video signal into an analog luminance signal in addition to a D/A converter for converting digital chrominance signal data into an analog chrominance signal.
FIG. 6 is a diagram showing a typical conventional 8-bit D/A converter and a peripheral circuit thereof. The conventional 8-bit D/A converter is used for converting digital luminance signal data output by a video camera apparatus into an analog luminance signal.
The conventional 8-bit D/A converter 100 shown in the figure is a current-output D/A converter for outputting a current representing digital data supplied thereto. The current output by the D/A converter 100 is converted into a voltage by using a resistor RL. In this way, the D/A converter 100 outputs an analog signal with a level representing digital data supplied thereto.
The digital data supplied to the D/A converter is typically 8 bits in width. From the 8-bit digital data, an analog luminance signal A having 256 tones representing the luminance signal data can be obtained.
By the way, when an analog luminance signal A is output from the 8-bit D/A converter 100 described above, it is necessary to add a synchronization signal Sync to the analog luminance signal A. In general, however, luminance signal data input from a block at a stage before the video camera apparatus or the like does not have a synchronization signal Sync added thereto. For this reason, a selector 102 and an adder 101 for adding a synchronization signal to the luminance signal data are provided on the input side of the 8-bit D/A converter 100. It should be noted that the block for generating the luminance signal data is not shown in the figure.
The adder 101 adds typically an offset of 64 for allowing for the synchronization-signal component to the input luminance signal data, outputting the result of the addition to a selector 102. The selector 102 has an input pin a for receiving the result of the addition from the adder 101 and an input pin b connected to the ground. The output of the selector 102 is switched from the input pin a to the input b or vice versa with timing determined by the synchronization signal Sync. In this way, the synchronization signal Sync is added to the sum of the luminance signal data and the offset value 64.
In order to supply 8-bit luminance signal data with a synchronization signal Sync added thereto to the conventional 8-bit D/A converter 100 in a circuit described above, a block at a front stage not shown in the figure provides the adder 101 with a digital value having tone levels 0 to 191 shown in FIG. 7A as luminance signal data. As described above, the adder 101 adds the offset value 64 to the luminance signal data to output digital data including luminance signal data with tone levels 64 to 255 shown in FIG. 7B to the selector 102. The selector 102 is switched from the input pin a to the input b or vice versa with timing determined by the synchronization signal Sync. As a result, luminance signal data with the synchronization signal Sync added at tone levels 0 to 64 as shown in FIG. 7C is supplied to the 8-bit D/A converter 100.
The luminance signal data with the synchronization signal Sync added thereto is supplied to the 8-bit D/A converter 100 for converting the luminance signal data into a current which appears as a voltage between the ends of the resistor RL. The voltage is the analog luminance signal A with a level ratio of 3:1 as shown in FIG. 7C where the level ratio is a ratio of the luminance-signal level to the synchronization-signal level.
As shown in FIG. 6, the digital luminance signal data is also supplied to a multiplier 104 for multiplying the digital luminance signal data by 4/3. That is to say, the multiplier 104 converts the luminance signal data with tone levels 0 to 191 shown in FIG. 7A into 8-bit luminance signal data at a full scale with tone levels 0 to 255 shown in FIG. 7E. The multiplier 104 then supplies the multiplied digital luminance signal data including no synchronization signal Sync to other equipment.
In spite of the fact that the digital luminance signal data supplied to the 8-bit D/A converter 100 along with the synchronization signal Sync added thereto is 8 bits in width, the digital value assigned to the luminance signal data is in a range of only 64 to 255. That is to say, the entire dynamic range 0 to 255 of the 8-bit D/A converter 100 can not be allocated fully to the digital luminance signal data.
As a result, the tone expression of the analog luminance signal output by the 8-bit D/A converter 100 is rough in comparison with a 8-bit D/A converter 100 with its entire dynamic range fully utilized, giving rise to a problem of a degraded picture quality.
In addition, there is also encountered another problem that the selector 102 and the adder 101 are required as peripheral components of the 8-bit D/A converter 100 for adding the synchronization signal Sync to the luminance signal data and, if the luminance signal data needs to be output as digital data as it is, the multiplier 104 is also required for converting the original luminance signal data into an 8-bit luminance signal at a full scale in the range 0 to 255.